The invention relates to a hydraulically powered valve actuator which is triggered to move between first and second stable positions by an electrically controlled pilot valve.
U.S. application Ser. No. 820,470 filed Jan. 14, 1992 and incorporated herein by reference discloses a resilient hydraulic actuator wherein the engine valve carries a single piston with opposed working surfaces which are alternately exposed to high pressure hydraulic fluid to shuttle the engine valve between first and second stable positions. When the main valve is in its first stable position (engine valve closed), a first fully charged spring chamber is isolated from a first working surface of the piston by a closed electrically actuated valve V.sub.1. Meanwhile, a second working surface of the piston is directly connected to a high pressure source via open valve V.sub.3, while a second spring chamber is connected to a lower pressure source via open valve V.sub.4 and disconnected from the working surface by a closed valve V.sub.2 .
The valves V.sub.2, V.sub.3, and V.sub.4 are on a common electrically controlled spool valve (pilot valve) and are therefore switched simultaneously so that the high pressure source is isolated from the second working surface (V.sub.3 closed), while the second spring chamber is isolated from the low pressure source (V.sub.4 closed) and connected to the first working surface of the piston (V.sub.2 open). High pressure from the first spring chamber then acts on the first working surface of the piston via a check valve to move the engine valve toward its second stable position, thereby increasing the pressure in the second spring chamber to provide damping. The momentum of the valve completes movement to the second stable position as pressure in the second spring chamber is maximized and pressure in the first spring chamber is minimized. Return movement is triggered by opening valve V, to release pressure from the first working surface of the piston back into the first spring chamber, followed by again switching the valves V.sub.2, V.sub.3, and V.sub.4 to complete the movement and latch the valve in the first stable position.
The actuator disclosed in U.S. Ser. No. 820,470 represents an important advance in electrically controlled hydraulically powered valves, insofar as it recognizes that compressibility of the hydraulic fluid may be used to create a spring for driving the valve and for damping its movement. However, two discrete solenoid actuated pilot valves are required, and the housing with its numerous internal passages is complex to manufacture.